ns: Reconnect NSVC after timeout

Currently the signal S_NS_ALIVE_EXP emitted by the NS layer if the
alive check has timed out too often is ignored. This prevents the PCU
from reconnecting to the SGSN if it has not been accessible for some
time.

This commit modifies nsvc_signal_cb to reset the NSCV if
S_NS_ALIVE_EXP is sent, so that the PCU continues to send NS RESET
message if that happened.

Sponsored-by: On-Waves ehf
diff --git a/src/gprs_bssgp_pcu.h b/src/gprs_bssgp_pcu.h
index 112a888..e0b8312 100644
--- a/src/gprs_bssgp_pcu.h
+++ b/src/gprs_bssgp_pcu.h
@@ -80,6 +80,7 @@
 		uint16_t lac, uint16_t rac, uint16_t cell_id);
 
 void gprs_bssgp_destroy(void);
+int gprs_ns_reconnect(struct gprs_nsvc *nsvc);
 
 struct bssgp_bvc_ctx *gprs_bssgp_pcu_current_bctx(void);